Systems, methods, and other techniques for improving the operation of computing systems are described. Some implementations include a computer-implemented method. The method can include intercepting, at an intermediary computing system, messages communicated between a web server system and one or more client computing devices. A subset of the intercepted messages can be selected that are determined to commonly relate to a particular web transaction. The method can identify an expression pattern that occurs in the subset of the intercepted messages, and can determine that the identified expression pattern matches a first pre-defined expression pattern from among a plurality of different pre-defined expression patterns. A status of the particular web transaction can be determined based on the first pre-defined expression pattern that matches the identified expression pattern occurring in the subset of the intercepted messages. Information can be stored that indicates the determined status of the particular web transaction.